1. Field of the Invention
The invention relates to a system and method for determining drive axle loading and estimating a vehicle""s sprung weight with improved accuracy.
2. Description of the Problem
Axle and total vehicle load are limited by law for trucks and desirable to monitor to avoid vehicle operating conditions which would be uneconomic or contribute to poor vehicle handling. To the present time, axle loading and vehicle weight have typically been determined by placing the vehicle on, or rolling the vehicle over, a scale. Commercial scales are often located at some distance from where a load is taken on. The need to then move the vehicle to the scale can be inconvenient. Should a vehicle prove overloaded with a subsequent need to return the vehicle to the loading point, the inconvenience factor is increased. Nor is a load necessarily constant during vehicle operation. Ice storms and heavy snow can contribute to overloading a vehicle. Loads may shift due to extreme operating conditions. Thus there is a need to provide updates of axle loading during vehicle use.
In response to the inconvenience and limited utility of using fixed scales to determine truck loads, and the interest in providing up to date information on axle loading, attempts have been made to provide on board vehicle weight and axle load measurement. One such technique estimates a vehicle""s mass using Newton""s law of acceleration. Force (torque) equals mass (the unknown) times acceleration. Full load engine torque may be obtained from a look up table and vehicle acceleration calculated from changes in vehicle speed over time. Vehicle mass is then easily calculated although several trials are often required. The accuracy of the result though depends upon engine operation meeting expected output, which can vary with age of the engine, the degree of streamlining of the vehicle, and other factors. The accuracy of the result may be further affected by external conditions such as weather and wind conditions, road slopes and the like.
Also known are systems relying on strain gauges fitted to leaf and coil springs, various types of displacement transducers, and of particular interest here, pressure transducers for air bladders used on height-leveled, air spring suspended vehicles. Vehicles equipped with air spring suspensions have used air gauges to monitor overall air pressure in air suspension springs. The gauge pressure has been equated to vehicle sprung weight for display to the driver. Alternatively, the pressure of individual air springs may be equated to loads on each of the axles. Some of these systems have provided for calibration against known loads to improve accuracy of the estimated weight.
One source of deviation from correct determination of vehicle weight and axle load is suspension system hysteresis. For a suspension system having air springs, the relationship of axle load to air pressure in the springs depends upon whether the load has previously increased or decreased. There are a number of sources of this hysteresis, some of which is designed into the system. Height-leveled, air spring equipped vehicles have a height control valve which will not open to allow the exhaust of air, or introduction of air, unless there is about a 900 lb. change in load. Spring bushings will exhibit some resistance to deformation. Stiction in the shock absorber allows some of the normally sprung load weight to be carried through the shock absorber.
Another source of transient error when using air spring pressure sensing to estimate axle loading is a change in vehicle speed. With acceleration, or deceleration, axle loading is transferred aft or forward, respectively. The error introduced by acceleration and deceleration can be substantial and immediately updating an axle load display to reflect the measured changes can become distracting to the vehicle""s operator.
Many contemporary commercial vehicles optimize automatic transmission start gears, shift point and running gear selection based on a trial and error seek process carried out over 25 to 30 trials involving sustained acceleration of the vehicle. In effect, the information is a byproduct of determining the vehicles"" mass, as described above. Data relating to the vehicles"" mass may be joined with an instantaneous velocity measurement to provide inputs into a look up table which returns an optimum gear choice for either best acceleration or economy operation. However, the need for repeated trials limits the utility of the system for vehicles such as dump trucks, delivery vehicles, and tankers subject to frequent changes in load.
The invention provides an axle load sensing system for a vehicle having an air bladder support system mounted between the frame of the vehicle and its axles. Axle load determination provides direct determination of vehicle load, which in turn is used to improve start gear, running gear and shift point optimization. At least a first pressure sensor provides air pressure readings for at least one air bladder of the air bladder support system. First and second transfer functions relate the air pressure readings to the weight carried by the air bladder support system. A transfer function indicator responsive to a direction of change in air pressure readings indicates which of the first and second transfer functions is to be interrogated to determine load. A processor connected to receive the air pressure readings and responsive to the transfer function indicator executes either the first or second transfer function using the air pressure readings as an input to return an estimated load carried by the air bladder support system. The transfer function indicator is responsive to prior increases in air bladder pressure for indicating interrogation of the first transfer function and to prior decreases in air bladder pressure for indicating interrogation of the second transfer function. The processor accumulates the returned estimates to provide a running indication of the load on the air bladder support system. To compensate for transient effects of vehicle acceleration on axle loads, returned estimates and the prior accumulated returned estimates are relatively weighted before being combined. Gear selection is improved with the weight data by providing gear choice look up tables for use by a transmission controller keyed to vehicle weight and speed.
Additional effects, features and advantages will be apparent in the written description that follows.